23 research outputs found
On the Phase Space of Block-Hiding Strategies in Bitcoin-like networks
We calculate the probability of success of block-hiding mining strategies in
Bitcoin-like networks. These strategies involve building a secret branch of the
block-tree and publishing it opportunistically, aiming to replace the top of
the main branch and rip the reward associated with the secretly mined blocks.
We identify two types of block-hiding strategies and chart the parameter space
where those are more beneficial than the standard mining strategy described in
Nakamoto's paper. Our analysis suggests a generalization of the notion of the
relative hashing power as a measure for a miner's influence on the network.
Block-hiding strategies are beneficial only when this measure of influence
exceeds a certain threshold.Comment: 28 pages, 7 figure
Superstring Theory on AdS_3 x G/H and Boundary N=3 Superconformal Symmetry
Superstrings propagating on backgrounds of the form AdS_3 x G/H are studied
using the coset CFT approach. We focus on seven dimensional cosets which have a
semiclassical limit, and which give rise to N=3 superconformal symmetry in the
dual CFT. This is realized for the two cases AdS_3 x SU(3)/U(1) and AdS_3 x
SO(5)/SO(3), for which we present an explicit construction. We also provide
sufficient conditions on a CFT background to enable a similar construction, and
comment on the geometrical interpretation of our results.Comment: 22 pages, harvmac; added reference
Superstrings on AdS_3 and Symmetric Products
Some properties of N=2 superstrings on AdS_3 x N are studied. Their spectrum
has the same pattern as 2-d CFTs in the moduli space of symmetric products
M^p/S_p, where p is associated with the number of long fundamental strings
required to construct the AdS_3 background, and M is the spacetime CFT
corresponding to short string excitations of a single long string. Worldsheet
operators associated with w long strings correspond to twisted sectors of
M^p/S_p.Comment: 43 pages, harvma
Towards observable signatures of other bubble universes
We evaluate the possibility of observable effects arising from collisions
between vacuum bubbles in a universe undergoing false-vacuum eternal inflation.
Contrary to conventional wisdom, we find that under certain assumptions most
positions inside a bubble should have access to a large number of collision
events. We calculate the expected number and angular size distribution of such
collisions on an observer's "sky," finding that for typical observers the
distribution is anisotropic and includes many bubbles, each of which will
affect the majority of the observer's sky. After a qualitative discussion of
the physics involved in collisions between arbitrary bubbles, we evaluate the
implications of our results, and outline possible detectable effects. In an
optimistic sense, then, the present paper constitutes a first step in an
assessment of the possible effects of other bubble universes on the cosmic
microwave background and other observables.Comment: 17 PRD-style pages including 13 embedded figures. Minor corrections
to figures 4 and 7 and added discussion in Sec. III.E.2 and
IIA moduli stabilization with badly broken supersymmetry
Scherk-Schwarz compactification in string theory can be defined as
orbifolding by an R symmetry, a symmetry that acts differently on bosons and
fermions. Such a symmetry can arise in many situations, including toroidal and
orbifold compactifications, as well as smooth Calabi-Yau spaces. If the
symmetry acts freely then for large radius there are no tachyons in the
spectrum. We focus mainly on stabilization by fluxes, and give examples with
all moduli stabilized where the coupling is small and the internal manifold is
large. Such models appear to be perturbatively stable with supersymmetry broken
at the Kaluza-Klein scale. These are interesting laboratories for a variety of
theoretical questions and provide models of a non-supersymmetric landscape.Comment: 18 pages, LATEX, no figures. several refs added to earlier literature
on Scherk Schwarz in string theory, discussion of smooth CY made more precis
The Spectrum of N=3 String Theory on AdS_3 x G/H
We continue the study of string theory on AdS_3 x SU(3)/U(1) and AdS_3 x
SO(5)/SO(3). We compute the spacetime spectrum of the N=3 supersymmetric dual
CFT using worldsheet techniques. The spectrum of chiral primaries coincides for
the two models. Unlike N=4 theories, the building block of the symmetric
product in spacetime (corresponding to a single long string) is not by itself
in the moduli space of a symmetric product.Comment: 18 pages, harvma
A note on Gauge Theories Coupled to Gravity
We analyze the bound on gauge couplings , suggested by
Arkani-Hamed et.al. We show this bound can be derived from simple
semi-classical considerations and holds in spacetime dimensions greater than or
equal to four. Non abelian gauge symmetries seem to satisfy the bound in a
trivial manner. We comment on the case of discrete symmetries and close by
performing some checks for the bound in higher dimensions in the context of
string theory.Comment: 15 pages, 1 figure, Late
"Double-trace" Deformations, Boundary Conditions and Spacetime Singularities
Double-trace deformations of the AdS/CFT duality result in a new perturbation
expansion for string theory, based on a non-local worldsheet. We discuss some
aspects of the deformation in the low energy gravity approximation, where it
appears as a change in the boundary condition of fields. We relate unique
features of the boundary of AdS to the worldsheet becoming non-local, and
conjecture that non-local worldsheet actions may be generic in other classes of
backgrounds.Comment: 21 pages, 2 figures, harvmac. v2: minor changes, references added,
version sent to JHEP. v3 minor correction